The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean.

An updated Atlantic hurricane outlook will be issued in early August, which begins the peak (August-October) of the hurricane season.

DISCUSSION

1. Expected Activity - 75% chance above normal, 20% chance near normal, 5% chance below normal

An important measure of the total seasonal activity is NOAA's Accumulated Cyclone Energy (ACE) index, which accounts for the collective intensity and duration of Atlantic named storms and hurricanes during the hurricane season. The ACE index is also used to define above-, near-, and below-normal hurricane seasons (see Background Information). A value of 117% of the median (Median value is 87.5) corresponds to the lower boundary for an above-normal season.

For the 2007 Atlantic hurricane season, the ACE index is expected to be in the range of 125% to 210% of the median. The upper portion of this range is above the 175% baseline that Goldenberg et al. (2001) used to define a hyperactive season. Based on this predicted ACE range, and on the 75% probability of an above-normal season, we expect a likely range of 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. This predicted ACE range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their expected ranges.

The vast majority of tropical storms and hurricanes are expected to form during August-October over the tropical Atlantic Ocean, which is typical for above-normal seasons. These systems generally track westward toward the Caribbean Sea and/or United States as they strengthen. Historically, above-normal seasons have averaged 2-4 hurricane strikes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea. However, it is currently not possible to confidently predict at these extended ranges the number or intensity of landfalling hurricanes, or whether a given locality will be impacted by a hurricane this season.

2. Expected Climate Conditions – Active multi-decadal signal, either ENSO-neutral or La Niña

The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean. Each of these factors is discussed below.

a. Expected continuation of active Atlantic hurricane era

Atlantic hurricane seasons exhibit prolonged periods lasting decades of generally above-normal or below-normal activity. These fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms first named in the main development region (MDR), which spans the tropical Atlantic Ocean and Caribbean Sea between 30oW-87.5oW and 10oN-21.5oN (Goldenberg et al. 2001).

Hurricane seasons during 1995-2005 have averaged 15 named storms, 8.5 hurricanes, and 4 major hurricanes, with an average ACE index of 179% of the median. NOAA classifies nine of the last twelve hurricane seasons as above normal, with seven being hyperactive. Only three seasons since 1995 have not been above normal. These are the El Niño years of 1997, 2002, and 2006.

This high level of activity contrasts sharply to the 1971-1994 period of generally below-normal hurricane seasons (Goldenberg et al. 2001), when seasons averaged 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, with an average ACE index of only 75% of the median. One-half of those seasons were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive. Time series' of key atmospheric wind parameters highlight the dramatic differences between these above-normal and below-normal hurricane eras.

The regional atmospheric circulation contributing to these long-period fluctuations in hurricane activity is strongly linked to the tropics-wide multi-decadal signal (Bell and Chelliah, 2006). A change in phase of that signal accounts for the transition in 1995 from the below-normal era to the above normal era . The multi-decadal signal is again a major factor guiding the 2007 outlook. Three key features of this signal that are associated with the current active hurricane era are: 1) a stronger West African monsoon system, 2) below-average convection in the Amazon Basin, and 3) warmer than average SSTs across the tropical Atlantic. Atlantic SSTs are presently 0.6oC above average in the MDR, and this warmth is likely to continue through the 2007 hurricane season.

Other ongoing regional aspects of the multi-decadal signal again expected during the 2007 hurricane season include 1) lower surface air pressure, and increased moisture across the tropical Atlantic, 2) an amplified ridge at upper levels across the central and eastern subtropical North Atlantic, 3) reduced vertical wind shear in the deep tropics over the central North Atlantic, which results from an expanded area of easterly winds in the upper atmosphere (green arrows) and weaker easterly trade winds in the lower atmosphere (dark blue arrows), and 4) weaker easterly winds in the middle and lower atmosphere, resulting in a configuration of the African easterly jet (wavy blue arrow) that favors hurricane development from tropical waves moving westward from the African coast.